Hopefully this pic explains it. Im not sure if that switches LED is rated for 12v or not, so youll have to determine that starting with a lower voltage. Itll either be hooked up by the 12v or 5v from your PSU, and use the negative from the HDD LED header.

The positive from the HDD header will go to the relay which will shunt (short, turn off etc) the negative to pole 87a to the LED switch turning it off.

So this one didn't work(no hard feelings). On the web page it says 22mm, I received a 16mm. When I called to talk to them about a replacement they said all they will be receiving is a 19mm from the manufacturer. Oh sad days!

Guess I'll just have to wait on that switch. But on the bright side the rest of my stuff is on its way!